Fix transition progress not applying to drawables

TaskLayerDrawable should reapply UI changes from the transition progress
when there is a new drawable. In addition, the transition should check
if the drawable in the front and back are the same (i.e. on
initialization when both are showing the empty drawable) so that it only
applies the front-drawable alpha.

Bug: 114136250
Test: Builds
Change-Id: I74836b5043da555358742ba0a3f46f170f590904